The present invention relates to a radio telephone having an array of manually operable buttons.
Keypads are known for supplying digital codes to processing equipment, such as mobile cellular telephones, in response to manual operation by a user. In order to convey digital codes to a processing device within the telephone, it is known to perform digital scanning in which, for an N.times.M switching matrix, a set of outputs energise each of N rows of switches in turn. For each energised row of switches, the columns are read as an M-bit word. Logic and associated software determine which row was energised when a column produces an active signal, therefore the active key which has been manually pressed can be identified from a word of M+N bits.
A disadvantage of this known technique is that for an N.times.M keypad, N outputs from, and M inputs to an input/output (I/O) device are required. Furthermore, the N outputs must be driven in sequence continually at a sufficiently high rate. Thus, N output buffers dissipate power most of the time. In addition, a software routine or logic circuit must be run in conjunction with the scan lines to decode the column information. These operations dissipate power which, in some applications, do not create a problem. However, in a mobile phone a major consideration is the rate at which the phone dissipates its local power supply. Thus, a problem with the known digital scan technique is that it may require power to be dissipated while the telephone is in its stand-by condition.
A further problem with the known digital scanning technique is that N+M connections are required between the I/O device and the keypad. In modern mobile phones, it is desirable to include as much processing power as possible within an application specific integrated circuit, so as to minimise assembly costs. Although it is possible to construct integrated circuits with a significant number of pins, 144 or 176 pins for example, it is desirable to minimise the pin count. Furthermore, the keypad itself may be mounted on a separate circuit board, commonly of a flexible type and the connection point between the main board and the keypad is vulnerable to breakage and dry jointing etc. Consequently, it is preferable to minimise the number of connections between the keypad and the main board. In addition a main board usually requires filtering components on each connection which, even in surface mount can occupy a large amount of board space and adds cost.